1. Field of the Invention
The present invention relates to a magnetic sensor which detects an external magnetic field and a position transducer using the magnetic sensor.
2. Description of the Related Art
As a magnetic sensor which detects an external magnetic field, there is known a one called flux gate type sensor. The flux gate type sensor includes a magnetic detector consisting of a core made of a high permeability material such as Nixe2x80x94Fe alloy and an exciting coil and sensor coil wound on the core. In the flux gate type sensor, when the exciting coil in the magnetic detector is driven with a sinusoidal or rectangular wave of tens kHz, the inductance of the core in the magnetic detector will be changed correspondingly to the strength of an external magnetic field incident upon the detection coil in the direction of the center axis of coil winding. In the flux gate type sensor, the strength of the external magnetic field is detected by converting a change of the core inductance to a voltage change.
It is known that with an optimum bias magnetic field applied to the magnetic detector, the flux gate type sensor will be able to detect even a faint magnetic field which is weaker than {fraction (1/10)}, for example, of the geomagnetism and also provide an output with an increased linearity, thus have a wider output dynamic range. Therefore, when the flux gate type sensor is used to detect an external magnetic field, the magnetic detector should desirably be applied with an optimum bias magnetic field.
As a magnetic sensor similar to the flux gate type sensor, there is also known a magnetic impedance effect type sensor using the so-called magnetic impedance effect (MI effect, hereunder). The MI effect type sensor has not a coil corresponding to the exciting coil in the flux gate type sensor since the detection coil in the magnetic detector is directly driven.
In the MI effect type sensor, when the detection coil in the magnetic detector is driven with a high frequency pulse whose rate is a few MHz to tens MHz and width is a few ns to tens ns or a sinusoidal wave whose pulse rate is similar to that of the high frequency pulse, the inductance component as well as actual resistance component of the detection coil will be changed due to the skin effect of the magnetic material, resulted from the high frequency excitation, correspondingly to the strength of an external magnetic field incident upon the detection coil in the direction of the center axis of coil winding. In the MI effect type sensor, the strength of the external magnetic field is detected by converting the change in impedance of the detection coil, which is a sum of the changes in inductance component and actual resistance component, to a voltage change and detecting the voltage change.
Since the strength of the external magnetic field is detected based on the impedance change of the detection coil, the MI effect type sensor has a higher detecting sensitivity than the aforementioned flux gate type sensor. It is also known that with an optimum bias magnetic field applied to the magnetic detector, the MI effect type sensor will be able have a higher detecting sensitivity and provide an output with a higher linearity, thus have a wider output dynamic range. Therefore, also when the MI effect type sensor is used to detect an external magnetic field, an optimum bias magnetic field should desirably be applied to the magnetic sensor.
For applying a bias magnetic field to the magnetic sensor of the magnetic sensor, it is well known to apply to the magnetic detector of a magnetic field from a permanent magnet disposed near the magnetic detector, and apply to the magnetic detector of a magnetic field developed by driving a bias coil wound on the magnetic detector.
However, in case a permanent magnet is used to apply a bias magnetic field to the magnetic detector, the magnetic field is easily changed due to non-uniformity in shape and material of the permanent magnet, which makes it difficult to select and position a permanent magnet for application of an appropriate bias magnetic field. Thus, this method is disadvantageous in that the manufacturing cost is increased and the magnetic sensor cannot work stably.
On the other hand, in case a bias coil is used to apply a bias magnetic field to the magnetic detector, since the strength of a magnetic field developed by the bias coil is determined by the value of a current supplied to the bias coil, an optimum bias magnetic field can easily be applied to the magnetic detector of the magnetic sensor by setting the value of the current supplied to the bias coil to an appropriate one. Therefore, this method for applying a bias magnetic field to the magnetic detector using a bias coil can be said to be very effective.
It should be noted that the output characteristic of the magnetic sensor depends upon an environment in which the magnetic sensor is used, for example, on the ambient temperature or the like around the magnetic detector, and it will shift as the ambient temperature changes. In the MI effect type sensor for example, when the ambient temperature around the magnetic detector changes, the impedance of the detection coil will be changed correspondingly to the temperature change and thus the output characteristic of the sensor will shift. When the output characteristic thus shifts, the optimum bias point to improve the output linearity of the magnetic sensor will shift.
If the bias coil is always driven with a constant current when a bias coil is used to apply a bias magnetic field to the magnetic sensor, it is not possible to follow up a shift of an optimum bias point due to a change of the ambient temperature or the like around the magnetic detector and apply an optimum bias magnetic field to the magnetic detector, possibly resulting in a reduced output linearity of the magnetic sensor.
It is therefore an object of the present invention to overcome the above-mentioned drawbacks of the prior art by providing a magnetic sensor which can well detect an external magnetic field even when the ambient temperature or the like around a magnetic detector of the magnetic sensor changes, by always applying an optimum bias magnetic field to the magnetic detector, and a position transducer using the magnetic sensor.
The above object can be attained by providing a magnetic sensor including a plurality of sensor units. Each of the plurality of sensor units includes a magnetic detector and a magnetic field developing means for applying a bias magnetic field to the magnetic detector. The magnetic sensor further includes means for driving and controlling the bias magnetic field developing means in the plurality of sensor units. In the magnetic sensor, the drive/control means monitors an output from the magnetic detector in one of the plurality of sensor units, and drives and controls the bias magnetic field developing means in the plurality of sensor units for the monitored magnetic detector to provide a constant output.
In the above magnetic sensor, the magnetic detectors of the sensor units detect an external magnetic field. At this time, the magnetic detectors are applied with bias magnetic fields, respectively, developed by the bias magnetic field developing means driven and controlled by the drive/control means. The drive/control means monitors the output from the magnetic detector in one of the plurality of sensor units, and drives and controls the bias magnetic field developing means of the plurality of sensor units for the monitored magnetic detector to provide a constant output.
Therefore, even when the ambient temperature or the like around the magnetic detectors changes, the bias magnetic field developing means can always apply an optimum bias magnetic field to the magnetic detectors to improve the output linearity.
Also the above object can be attained by providing a position transducer using the above-mentioned magnetic sensor. The position transducer includes a magnetic field developing means for developing a magnetic field whose strength and direction are changed correspondingly to a position of the magnetic field developing means, and a plurality of sensor units movable relatively to the magnetic field developing means. Each of the plurality of sensor units includes a magnetic detector and a bias magnetic field developing means for applying a bias magnetic field to the magnetic detector. Also, the position transducer further includes means for driving and controlling the bias magnetic field developing means in each of the sensor units, and means for detecting a moved position of each of the plurality of sensor units relative to the magnetic field developing means on the basis of an output from the magnetic detector in each of the plurality of sensor units. In this position transducer, the drive/control means monitors the output from the magnetic detector in one of the plurality of sensor units, and drives and controls the bias magnetic field developing means in each of the plurality of sensor units for the monitored magnetic detector to provide a constant output.
In the above position transducer, when the plurality of sensor units moves in relation to the magnetic field developing means, a magnetic field corresponding to the position of the magnetic field developing means will be incident upon the plurality of sensor units. The magnetic field from the magnetic field developing means and incident upon the plurality of sensor units will be detected by the magnetic detectors in the sensor units. At this time, the magnetic detectors are applied with bias magnetic fields, respectively, developed by the bias magnetic field developing means driven and controlled by the drive/control means. The drive/control means monitors the output of one of the magnetic detectors in the plurality of sensor units, and drives and controls the bias magnetic field developing means in the plurality of sensor units for the monitored magnetic detector to provide a constant output. Therefore, even when the ambient temperature or the like around the magnetic detectors changes, the bias magnetic field developing means can always apply optimum bias magnetic fields to the magnetic detectors. The outputs of the magnetic detectors in the plurality of sensor units are supplied to the detecting means. The detecting means detect relative positions of the plurality of sensor units to the magnetic field developing means on the basis of the outputs from the magnetic detectors.
According to the present invention, even when the ambient temperature or the like around the magnetic detectors for detecting a magnetic field changes, an optimum bias magnetic field can always be applied to the magnetic detectors to detect the external magnetic field with a higher accuracy.